Electronic timepieces with an analogue display are known which include an internal digital timekeeper in addition to the hands rotating above the dial. This internal timekeeper is clocked by the same electric pulses that control the forward movement of the hands. In principle, the hands and the timekeeper move forward, in a synchronous manner. The use of this type of internal timekeeper in multifunction watches where the same hands are used for indicating the time or a second piece of information, such as for example an alarm time, is also known. In fact, an internal timekeeper is necessary if one wishes to be able to continue counting the time that has elapsed while the hands are occupied with displaying the second piece of information. When the hands then return to their time display function, the data contained in the internal timekeeper allows them to reset to the correct time.
However, if one wishes an application like that just described to be satisfactory, one must be able to prevent the appearance of any difference between the time displayed by the hands of the watch and the time given by the internal timekeeper. It is known that this difference may occur for example if the watch has experienced a shock or because of an electromagnetic or even a mechanical disturbance (dust in the gear train for example). Because of these disturbances, the motors of some timepieces lose steps. Every analogue timepiece is thus liable to show a difference between the counting of the control pulses and the angular position of the hands. If this difference is not corrected in time, it may increase to the point where it results in totally erroneous indications. Moreover, in multifunction timepieces, the hands must be able to move not only forwards, but also backwards, depending upon variations in the quantity to be indicated. Further, the hands of a multifunction timepiece must be capable of changing position quickly forwards or backwards, when the function is changed. In order to satisfy these constraints, the hands of multifunction timepieces are generally each driven by their own motor. Consequently, instead of the single internal timekeeper described hereinbefore, multifunction timepieces generally comprise a counting/count-down circuit for the control pulses of the motor of each hand. It will be understood, moreover, that the motors of a multifunction timepiece have to satisfy considerably greater stresses. In these conditions, the risk of any difference, or in other words desynchronization of the hands, is also considerably higher for a multifunction timepiece than for any other.
In order to overcome the problems that have just been described, it is known to complete the motor control pulse counting/count-down with detection of the real position of the hands. EP Patent No. 0 841 538, in particular, discloses a timepiece that includes a watch movement driving an analogue display. This movement includes a wheel secured in rotation to one of the hands, and a magnetic sensor, which is for detecting the position of the wheel. In order to allow such detection, the plate of the wheel is covered in places with a magnetic film that defines a complex pattern, such that the magnetic sensor supplies a 1 or 0 signal depending upon whether or not it is opposite a place covered with magnetic film. The timepiece further includes electronic means for determining, from the 1 and 0 sequence, whether the position of the hand actually corresponds to the theoretical position. The device disclosed in this latter document has some drawbacks. In particular, detection of the loss of a step can only occur retrospectively. Moreover, the operation of covering the plate of a wheel with a pattern cut into a magnetic film can only increase manufacturing costs.
EP Patent No. 0 952 426 discloses another solution, which addresses the same technical problem. This document also discloses a timepiece movement comprising a wheel secured in rotation to one of the hands. This wheel is formed of a plate that has at least one aperture located in the intermediate region between the axis of rotation and the circumference. The timepiece further includes a device for detecting the angular position of the wheel. This device includes an inductive or capacitive sensor, arranged such that it is located directly above the aperture in the plate when the wheel is occupying a particular position. The sensor is sensitive to the variation in the quantity of metal located in immediate proximity. The amplitude of the signal sensed by the sensor thus varies depending upon whether it is located opposite a full segment or, conversely, the aperture in the plate of the wheel. The device further includes a memory for storing the signal amplitude after each step and electronic data processing means for determining retrospectively, from the data stored, the instant at which the aperture in the plate was located directly above the sensor.
The prior solution that has just been described also has some drawbacks. In particular, the wheel whose angular position is detected rotates at the same rhythm as a hand. If the plate of the wheel only has a single aperture, the angular position of the wheel, or of the hand, is only sensed once per revolution. It is known that in usual timepieces, a hand is expected to make a minimum of sixty steps per revolution. It may therefore be necessary to wait 60 steps before being able to verify the position of the hand. Moreover, the rotational amplitude of a hand corresponding to a single step is very limited (six degrees at best). In such conditions, there is only a tiny change in the intensity of the signal detected by the sensor from one step to the next, and it is not possible to detect reliably the instant at which the aperture passes directly above the sensor. This is why, according to the prior art, determination of the position of the hand does not occur in real time, but retrospectively.
EP Patent No. 1 662 343 discloses a timepiece comprising an optical sensor for detecting a reference position. Like the preceding timepieces, this one comprises a timepiece movement including a wheel secured in rotation to one of the hands. This wheel meshes with the pinion of an intermediate wheel set, which is itself driven by a stepping motor. The gear ratios are such that the intermediate wheel set completes an integer number of rotations during each revolution of the hand. The wheel secured to the hand and the wheel of the intermediate wheel set partially overlap. These two wheels are each formed of a plate having an aperture made in the overlap zone of the two wheels. In such conditions, the apertures of the two wheels are in a superposed position exactly once during each revolution of the wheel secured to the hand. This periodic coincidence of the position of the two apertures defines the reference position of the hand. An optical sensor is also arranged at the place where the positions of the two apertures coincide. The optical sensor is formed of a light source and a photosensor arranged on either side of the two wheels such that the light from the light source cannot reach the photosensor, except when the two apertures are located in the extension of each other. The signal provided by the optical sensor is used to determine the instant that corresponds to the passage of the hand into the reference position thereof.
Unlike the case of the wheel secured to the hand, each step of the motor produces a relatively large movement of the intermediate wheel aperture. In such conditions, the signal intensity detected by the sensor passes practically from all to nothing between one step of the motor and the next. It is thus, theoretically, possible to detect the exact instant at which the hand passes into the reference position thereof in real time. The fact remains, however, that the reference position is only detected once per revolution of the hand. As with the timepiece movement of the preceding document, it may thus be necessary to wait for the motor to complete 60 steps, or more, before being able to verify the position of the hand.
The abstract of JP Patent No. 58131583 discloses a timepiece which includes a timepiece movement with hands. The timepiece includes a device for detecting the angular position of one of the hands. This device includes a magnet secured to the hand, and a series of Reed contacts arranged around the circumference of the dial and separated from each other by an interval corresponding to two steps of the hand. If it is assumed that the hand completes 60 steps per revolution, the timepiece will have to include 30 Reed contacts. This feature leads to an increase in manufacturing costs and is suited only to relatively large sized timepieces.
The abstract of JP Patent No. 2003107174 discloses a timepiece wherein the stepping motor is provided with electronic detection means for verifying, after each control circuit pulse, that the motor has in fact completed a step. This solution also has some drawbacks. Indeed, the electromagnetic field in the timepiece motor is generated both by the electric pulses powering the coils and by the magnet rotating with the rotor. In such conditions, the loss of one step by the motor leads only to a relatively moderate variation in the signal detected by the electronic means. Thus, the solution proposed in this prior document requires the implementation of a sufficiently sensitive and thus relatively sophisticated detection system. Moreover, any electromagnetic interference occurring in proximity to the watch is liable to cause errors in the detection system.